February 25 through March 2, I had the great pleasure of floating down the last 51 miles of the Grand Canyon, something I did in 1966. I joined the end of a wonderful adventure run under the auspices of Dave Mortensen and Tom Martin that involved reincarnating, then rowing, three replicas of wooden dories first built and used in the 1950's, part of the amazing and inspiring history of individual exploration of the Canyon begun by Powell -- (and continuing today, even in the midst of the misguided NPS-comm-op management of river traffic). This contemporary adventure is being documented on
http://historicriverboatsafloat.org/Aside from my gratitude for being taken on this 5-day jaunt, I want to comment on the reach's exciting geology, and in a second entry, on the western boundary-that-ought-to-be for Grand Canyon National Park.
For three-fourths of a century, the Colorado has come down the Grand Canyon and in its last 40 miles, interacted with the level of Lake Mead, as it is dictated by the Southwest's widely fluctating precipitation and modulated by the operation of Glen Canyon Dam. For the first third of that time, there was no Glen to trap the major silt-load, so much of that settled in the river's mainstream and back up into its side canyons. For the last few years, a time of persistent drought, the level of Mead has dropped severely, even as Glen is operated to emit its heavily water-law laden, but virtually siltless, flows.
Lake Mead, at about 1100' elevation, is about 100' down, and the Colorado has a river flow now well below the Grand Wash Cliffs. And that flow cuts into and carries away the silts of decades, as in this photo, just beyond the Cliffs. (photo by Kathy Darrow)
Seems to me, since 1930, this deposition and erosion, this laying down of layers and cutting through them, makes the last 40 miles of the Canyon its most active geologic section. Flash floods remake rapids here and there, but this is a continuous strip of add-and-subtract, as shown by all those layers in the photo. Unconsolidated layers, so the "cliff" keeps crumbling away, even as one sits and watches.
And from one extreme of the drought-and-flood "cycle" to the next, what is happening under the surface? Were the depositions fairly evenly spread? Did they push the water, when the level was high, to and fro across the flats, ignoring the pre-Hoover channel? Is that channel now filled? How does the river choose how wide and deep to flow now that the gradient is so much steeper than reservoir-normal? What would cross-sections of the channel have looked like over the years? Then there is all that thick invasive vegetation, much now way above the water that fed it. What and who lives there, and how is it all adapting?
Although the Park Service lets the Hualapai buzz its tourist masses up and down the river a bit, no upstream traffic is allowed from Pearce Ferry, so perhaps there is no geologic study going on or practicable of the last 40 miles. Not that the area is devoid of people. The traffic of helicopters in the few miles of the river corridor near Quartermaster Canyon is even more continuous than the silt falls from the banks. One trusts that the Hualapai are becoming wealthy; there certainly are a lot of Las Vegas tourist dollars represented in all the mechanized hurry-scurry of these massed visits to the rim-view platform, and then helicoptered down to river level and back. A good reminder of what our National Park System, imperfect as it may be, is supposed to not be about.